Electronic circuitry is frequently employed in situations wherein it is subjected to adverse environmental conditions and/or to physical stress including shock and mechanical abuse, as well as to severe point pressure and abrasion. In order to protect such sensitive circuitry, it has become known to embed circuitry in an encapsulation composition. The materials employed in such encapsulation compositions must possess high electrical resistivity and should exhibit high arc resistance and track resistance in order to protect circuitry from moisture, physical shocks, and the like.
Among the materials which have been found to exhibit highly desirable properties for encapsulation purposes are copolymers of ethylene, propylene and a nonconjugated diene (i.e., "EPDM"). Thus, U.S. Pat. No. 3,974,132 to Valdiserri discloses the insulation and coating of electrically conducting wire with EPDM which is crosslinked with a trialkyl phosphate. However, several disadvantages are associated with the use of EPDM polymers such as those disclosed in this patent.
Primary among these disadvantages is that in order to cure such EPDM compositions a curing agent (typically a peroxide) is required, and, consequently, heat must be applied to cause curing to be initiated. Because much electronic circuitry is temperature sensitive, the thermal shock encountered by the curing of such polymers may damage the circuitry to be protected.
Moreover, the EPDM employed in the Valdiserri and other similar processes is a high molecular weight material which must be pelletized. Consequently, in order to adapt the embedding composition to the configuration of the circuity to be protected, such pelletized EPDM must be simultaneously subjected to heat and pressure. The thermal and mechanical shocks resulting from such heat and pressure may cause damage to sensitive electrical circuitry.
In addition, such pressure and temperature requirements prohibit the accomplishment of on-site encapsulation, e.g., for the protection of splices or other on-site repairs.
In order to overcome these difficulties, copending U.S. patent application No. 787,870, filed Oct. 16, 1985, discloses certain liquid epoxy-modified embedding compositions which are pourable and which, due to the presence of such epoxy groups, may be cured at ambient temperature. Moreover, because such compositions are liquids (i.e., they have a definite volume and will assume the shape of their container) the application of pressure to cause them to adapt to the conformation of the circuitry to be encapsulated is not necessary.
The most preferred of these compositions are those which exhibit a low Brookfield viscosity (and which are thus more readily poured and formed into appropriate configuration at room temperature) and those exhibiting a high viscosity index (which are thus more readily pourable at low temperatures).
It has now been found that those encapsulation compositions which are formed employing the ethylene/alphaolefin/(optionally) nonconjugated diene polymers disclosed herein as intermediates (which copolymers possess vinylidene-type unsaturation) will exhibit a desirably high viscosity index (of at least about 75), coupled with an unexpectedly low Brookfield viscosity relative to identical compositions formed from other polymers having a similar composition and molecular weight but not exhibiting vinylidene-type unsaturation.
Moreover, it has also been unexpectedly found that encapsulation compositions based upon copolymers having vinylidene-type unsaturation exhibit increased adhesion to wire relative to compositions based on similar copolymers not possessing vinylidene-type unsaturation.
South African Patent Application No. 824,858 discloses one process by which the polymers of this invention may be prepared. However, the copolymers actually disclosed in this Application are all of too high a molecular weight to be liquid materials. While low molecular weight (Mn=5,000) polypropylene homopolymer was prepared, polypropylene is comparatively undesirable for use in such pourable encapsulation compositions because (as is indicated by its relatively low viscosity index of about 50) such material is difficult to pour at low temperatures.
Sinn et al, in U.S. Pat. No. 4,404,344, disclose the use of a halogen-free catalyst which may be employed to produce the compounds of this invention, but show only the production of solid, unpourable high molecular weight polymers.
John A. Ewen, "Mechanisms of Stereochemical Control in Propylene Polymerizations with Soluble Group 4B Metallocene/Methylalumoxane Catalysts", Journal American Chemical Soc., Vol. 106, pp. 6355-6364 (1984) discloses that polypropylene made by a dicyclopentadiene zirconium dichloride/methylalumoxane polymerization catalyst exhibits some vinylidene unsaturation (in Table XII). However, as is noted above, polypropylene is less desirable as a base polymer due to its comparatively low viscosity index.
Thus, it is to be noted that none of the above publications disclose the actual making the compounds of this invention nor do they provide any motivation for doing so. The lower Brookfield viscosities and increased adhesion to wire exhibited by electrical encapsulation compositions based upon the intermediates of this invention (relative to compositions based on similar copolymers not having vinylidene-type unsaturation) is completely unexpected from such disclosures.